The present invention is employed in a rotating polygon-based optical system. A known rotating polygon multi-beam ROS scanner system is described below, for easier understanding of the technical discussion, in Prior Art FIG. 10. It should be appreciated that the functions described below equally apply to many polygon-based systems, independently of the number of light sources used.
FIG. 10 shows a pair of sagittally offset laser diodes 31 and 32. The beams 41 and 42 emitted by laser diodes 31 and 32 are collimated by collimator 33 (lens L1). A sagittal aperture 34 follows the collimator to control the F/#, which in turn controls the spot size created by the beams. The input cylinder optical element 35 (lens L2) focuses the beams 41 and 42 on the surface of a polygon facet 36 of the rotating polygon. After reflecting from facet 36, beams 41 and 42 pass through the f-.THETA. (f-theta) lens 37 (lens L3). The main function of the f-.THETA. lens 37 is to provide focusing in the tangential meridian and control the scan linearity, in terms of uniform spot displacement per unit angle of polygon rotation.
Subsequently, the motion compensating optical element (MCO) 39 reimages the focused beams 41 and 42 reflected from polygon facet 36 onto the photoreceptor (PR) plane 40 at a predetermined position, independently of the polygon angle error or tilt of the current facet 36. The MCO can consist of a toroidal surface in the f-.THETA. lens, a post-polygon cylinder mirror or a post-polygon cylinder lens. Such compensation is possible because the focused beams are stationary "objects" for the f-.THETA. lens 37 and the MCO 39. Although, due to polygon tilt, or wobble, the beams 41 and 42 are reflected to different positions of the post-polygon optics aperture for each different facet of the rotating polygon, the beams 41 and 42 are imaged to the same position on the PR plane 40. It should be appreciated that in Prior Art FIG. 10, the chief exit rays from the MCO are not telecentric. That is, the chief exit rays are not parallel with the system axis 38.
Heretofore, a number of patents and publications have disclosed ROS-based recording systems, the relevant portions of which may be briefly summarized as follows:
U.S. Pat. No. 3,750,189 to Fleischer (issued Jul. 31, 1973) discloses a ROS system including a laser whose single-beam, modulated output is collimated and focused onto the facets of a rotating polygon. The reflected beams pass through an f.THETA. lens lens system and are focused in the scan direction on the surface of a moving photoreceptor. A start-of-scan photosensor is located in the scan plane.
U.S. Pat. No. 4,253,102 to Kataoka et al. (issued Feb. 24, 1981) teaches an optical information recording apparatus employing a semiconductor laser array. The laser array is positioned in an inclined condition so as to reduce the spacing between light spots on the surface of a recording medium.
U.S. Pat. No. 4,390,235 to Minoura (issued Jun. 28, 1983) discloses a multi-beam scanning apparatus for scanning a surface with a plurality of beam spots modulated independently of one another. Included in the system is an anamorphic afocal zoom lens which has the function of changing the angular magnification, resulting in a proportional change in the spot size as well.
U.S. Pat. No. 4,404,571 to Kitamura (issued Sep. 13, 1983) teaches a multi-beam recording apparatus employing a light source with an array of laser light sources that may be inclined with respect to a straight line normal to the optical axis of the system. Also disclosed is a beam detector suitable for the detection of individual beams passing through a screen plate associated therewith.
U.S. Pat. No. 4,474,422 to Kitamura (issued Oct. 2, 1984) discloses a multi-beam optical scanning apparatus employing a collimating portion positioned subsequent to a polygon reflector.
U.S. Pat. No. 5,300,962 to Genovese (issued Apr. 5, 1994) teaches a raster output scanner for multicolor printing wherein adjustment of the relative intensity of sub-beams in a writing beam may be used to control the position of the writing beam on a photoreceptor.
In "Self-Focusing ROS ", Xerox Disclosure Journal, Vol. 18, No. 2, (March/April 1993), pp. 151-52, Robson teaches a single-beam ROS suitable for producing a small spot size that may be adapted for use with microfilm imaging.
The main shortcoming of the systems shown in FIG. 10 and described in the related patents and publications is an inability to produce focused scan lines with sufficient line separation depth of focus to enable the use of two or more lasers, with an interlace of greater than one. This constraint to the use of an interlace of one dictates that these ROS designs will be radiometrically inefficient, since significantly more truncation of the beam is required with an interlace of one than with a higher order interlace.
"Line separation depth of focus" represents the distance along the optical axis over which the line separation is within a specified tolerance of the nominal value. This line separation depth of focus may also vary along the scan line. "Differential bow" is the variation in line separation along the scan line. Thus, differential bow is a special case of line separation, which is the more general imaging parameter. Insufficient line separation depth of focus, and therefore differential bow depth of focus, are primarily attributable to the angular deviation between the chief rays and the system axis between the MCO and the PR image plane. This angular deviation makes it difficult to maintain the line separation and differential bow over a workable depth of focus.
In rotating polygon, multiple spot ROS-based xerographic copiers and printers, it is necessary to accurately maintain the required line separation and to minimize the differential bow. Moreover, it is desirable to maximize the depth of focus for the line separation and the differential bow so as to reduce the critical tolerances for mechanical components within the copier or printer. In a preferred design for a ROS-based system, the system common depth of focus (system common DOF) is maximized, where the system common DOF is characterized as the depth- of-focus over which all performance parameters are met. More specifically, the performance parameters are intended to include at least the following five factors: (1) scan and cross-scan spot size; (2) wobble; (3) differential bow; (4) line separation; and (5) scan linearity. Maximizing the system common DOF means to simultaneously maximize the depth-of-focus for all five listed parameters.
The five performance parameters may be further described as follows:
"Spot size" is typically measured at the Full Width Half Maximum (FWHM) or at the 1/e.sup.2 point of the Gaussian Beam. The resolution and image processing requirements of the system determine the desired spot size. Assuming that a 600 spot per inch (spi) system is being designed and that the FWHM spot size is to equal the raster spacing, the desired FWHM spot size is: (1 inch/600 spots)(25.4 mm/inch)(1000 .mu.m/1 mm)=42.3 .mu.m round spot Hence, the spots would overlap at the FWHM in both the scan and in the cross-scan directions. Variations in the desired spot size occur depending on whether or not the spot is pulse width modulated. For gray writing, an elliptical (anamorphic) spot may be desired (typically narrower in the scan plane than in the cross scan plane). With specially designed electronics the spot may be controlled by pulse width modulation to the desired size within the raster spacing and thus the desired gray level.
"Wobble" is the unequal spacing of successive scanlines in the process direction at the image plane. Wobble appears to the human eye as banding in a final print. The presence of wobble can be quite disturbing if it occurs within a frequency range over which the eye is most sensitive (typically 0.5 to 2.0 cycles/mm). Therefore, wobble correction is essential over this frequency range in ROS designs. Wobble is directly related to the amount of pyramidal error in the polygon facets. A physical (mechanical) facet tilt of .+-.0.5 minute (30 arcseconds) produces a .+-.1 minute (60 arcseconds) of optical tilt.
"Bow" is a measure of the curvature in the cross-scan direction of the scan line from one end of the scan to the other. Bow may be calculated by taking the average of the cross-scan heights at the extreme ends of the scanline then subtracting the cross-scan height at the center of scan. In a multiple diode system, each diode source has its own bow curve. It is the maximum difference in the bow curves between the multiple diodes in a given system that defines the "differential bow". Typically, the bow specification in black-only machines may be quite large, on the order of 150-200 .mu.m. However, the differential bow specification must be held much tighter.
The required "line separation" is dependent on the desired interlace factor. For a scan line interlace factor of 3 for 600 spi raster spacing the line separation is 127 .mu.m.
The optical design must achieve f-.THETA. correction in the optics to ensure the "scan linearity." Scan linearity is the measure of how equally spaced the spots are written in the scan direction across the entire scanline. Typical scan linearity curves start at zero position error at one end of a scan having a positive lobe of position error, cross the center of scan with zero position error and then have a negative lobe of position error toward the other end of the scan. Scan linearity curves may have image placement errors of zero at several locations across the scanline. Ideally, the curve would be at zero across the entire scanline.
Although a multi-beam, laser diode based ROS is viewed as a most powerful technology for high quality, high throughput xerographic printing, the necessity for high tolerance mechanical, systems to eliminate or control the above effects within the xerographic engine is a barrier to increased speed and reduced cost for such systems. Accordingly, the present invention is directed at a ROS system that not only achieves the desired line separation, enabling higher throughput levels, but does so while maintaining substantial system common depth of focus, thereby reducing the tolerance for other xerographic engine components, such as the photoreceptor and facilitating the alignment process.
In accordance with the present invention, there is provided a multispot optical scanning system for exposing a surface of a photoreceptor, comprising
a post-scanning optical system for placing the beams reflected from the light reflecting surface of said reflective scanning member in a path telecentric with an optical axis of the post-scanning optical system prior to striking the surface of the photoreceptor so as to maximize the system common depth of focus about a focal plane defined by the photoreceptor surface, said post-scanning optical system including